The present invention relates to inspection of an open-face cell structure bonded to a substrate and, more particularly, to an apparatus and method for inspecting an open-face honeycomb cell structure of the type commonly used in the aircraft and aerospace industry, including use in gas turbine engines for aircraft propulsion as a seal between non-rotating outer casings and inner rotating components, such as gas turbine engine blades.
Rotating components of a gas turbine engine, such as the fan section, turbine section and compressor section of the engine are typically surrounded by a casing or lining having an interior portion formed of a cell structure having a multiplicity of cells which are typically six sided. These cell structures are commonly referred to as honeycomb because of the distinctive shape of the cells. The interior of each of the hexagonal cells is open and one end of the cell structure is bonded or brazed to a substrate with the open end facing the rotating component of the gas turbine engine.
The honeycomb structure in conjunction with teeth or fingers extending radially from each of the rotating components or blades creates a seal for more efficient operation of the gas turbine engine. The honeycomb structure is abradable to accommodate slight radial growth of the rotating component caused by thermal and centrifugal forces acting upon the rotating component. Thus, the radially extending blade fingers may peel away a minute layer of the honeycomb structure. Therefore, it is important that the honeycomb structure be properly or well bonded to the substrate to prevent a large portion of the honeycomb structure from being peeled from the substrate if the radially extending fingers contact the honeycomb seal which could result in further damage to the engine. Additionally, there should be no leaks between the adjacent honeycomb cells to provide proper sealing and air flow through the engine and therefore efficient engine operation.
Additionally, the honeycomb cells should not be plugged by braze or bonding material because this additional bonding or brazing material within a cell could break off one of the radially extending fingers from a rotating blade resulting in further damage to the engine and also loss of the air seal at this section of the engine resulting in decreased engine efficiency and performance.
Thus, it is important that the honeycomb cell structure be inspected for disbonds between the cell structure and the substrate to which it is bonded or brazed and also for plugged or partially plugged honeycomb cells.
A current inspection apparatus and method for inspecting a honeycomb cell structure is a visual inspection method which includes fixturing the part under an intense light source and viewing each of the individual honeycomb cells through a microscope or similar device for magnifying each of the cells. The operator thus visually determines if a disbond is present or if plugged cells exist. This apparatus and method is dependent upon the skill and vitality of the inspector or operator who must individually observe each of the honeycomb cells. Thus, this method is slow and inefficient and fatigue of the operator can result in inspection errors. While this method may possibly be used to inspect honeycomb cells having diameters of about 1/16" and 1/32", it is impractical for inspecting smaller honeycomb cells as small as about 1/64" in diameter.
Another method for inspecting the honeycomb cell structure involves filling each of the cells with a solvent, such as trichloroethane or the like. The honeycomb component is then rotated with the open face open to the ground. If the cell is properly bonded and sealed to the substrate, the solvent will remain within the cell. If a disbond exists, air will be allowed into the cell through adjacent cells and the solvent will fall out. This method has disadvantages in that plugged cells cannot be detected and honeycomb cells as small as 1/64" in diameter hold the solvent within the cell, even if there is a disbond, because of the surface energy within the smaller cells. Additionally, cleanup of the honeycomb structure after the inspection is necessary and the trichloroethane must be properly handled and disposed of. New environmental standards are also seeking the restrictive use of trichloroethane and similar materials hazardous to the environment which may be utilized with this method.
It is accordingly a primary object of the present invention to provide a novel apparatus and method for inspecting the bonding between a cell structure and a substrate which is not subject to the foregoing disaidvantages.
It is a further object of the present invention to provide an apparatus and method for inspecting the bonding of a cell structure which is reliable, minimizes operator fatigue and operator errors and substantially reduces the inspection time over current inspection methods.
It is a further object of the present invention to provide a novel apparatus and method for inspecting the bonding between a cell structure and a substrate which is environmentally safe.
It is yet another object of the present invention to provide an apparatus and method for inspecting the bonding between a cell structure and a substrate which can be incorporated into an automated system.
These and other objects of the invention, together with the features and advantages thereof, will become apparent from the following detailed specification when read with the accompanying drawings in which like reference numerals refer to like elements.